The subject invention relates in general to microphones or transducers and in particular to microphones or transducers used to provide a feedback signal upon being impacted by ink droplets.
In the technology of ink jet printing by the use of liquid droplets propelled from a fine orifice, an element of the printer that must be considered is the means by which proper droplet charging synchronization can be detected. The synchronization can be detected by using a very narrow test charging pulse applied to the droplet charging electrode as shown in U.S. Pat. No. 4,012,745 to brown et al, having a common assignee as the subject application. A sequence of such pulses applied in a proper phase relation to the droplet formation can result in the test charging and subsequent downward deflection of the droplets. This downward deflection can produce a change in the impact upon the surface of the droplet catcher microphone element. One of the difficulties in the existing art is the large amount of mechanical noise generated in a non-impact ink jet printer due to the air flow in which the ink droplets travel to reduce the Reynolds number. This noise interferes with the desired signals produced by the impacts of the downward deflected droplets upon the surface of the microphone catcher. Electrical noise is also a problem if the microphone catcher is placed close to the charged deflection plates. Special electronic circuit means must be used to filter the electrical signals to produce an acceptable siganl at the microphone amplifier.
The U.S. Pat. No. to Keur et al, 3,465,351 discloses a drop detector having a piezo-electric material body which generates a pulse output responsive to every drop which falls upon the piezo-electric material.
IBM Technical Disclosure Bulletin, Vol. 15, No. 5, October 1972, discloses a drop-sensing device using a piezo-electric material sandwiched between a support structure at its upper surface and a deflecting member at its lower surface. A sensing result from the piezo-electric material is applied to a circuit which alters the phase position of droplet generation and voltage at the charging electrodes of the ink jet printer.
There are a number of desirable features for a droplet microphone not shown by the prior art patents or publications. For example, it is desirable to combine an ink catcher and a microphone detector which minimizes splash of the conductive ink and also shields the transducing piezo-electric elements from the ink; if a cantilever arm bimorph is used, its length can be selected with the resonant frequency of the system in mind; and a diaphragm element can be used to receive the impact force of the moving droplets and can be disposed at an angle relative to the droplet path to minimize the splash of the ink used.